The most widely used black pigment is carbon black (C. I. Pigment Black 7). Carbon black is inexpensive and has excellent performance characteristics, such as stability to light and weather, high colour strength, jetness and neutral grey shades when mixed with white pigments such as titanium dioxide (C. I. Pigment White 6), but also serious disadvantages, such as problematic dispersibility of very fine particles, excessive electric conductivity and high absorption of long-wave ultraviolet radiation (UV-A), which is a massive obstacle to producing thick black matrices for colour filters by customary resist processes.
Compositions used for black matrices are required to have a low viscosity even at high pigment content and high sensitivity to curing by long-wave ultraviolet radiation even at high layer thicknesses (for example 2-5 μm). To ensure the very small pixels (for example≦10 μm) needed for high-definition screens, the resist has to provide very high definition. Cured black matrices should moreover have low roughness and also a high, ideally uniform absorption of the entire visible light coupled with negligible electric conductivity—and these properties shall have to withstand high thermal stress. Requirements further include high stability and good wettability in respect of solvents, and the functioning of the colour filter must not be impaired, for example by scattered light or “cross-talk”.
JP 2004/168 963 discloses a carbon black pigment modified by wet oxidation, the conductivity of which is said to be very low. However, actinic curing even of layers just ˜1 μm thick requires high irradiation and a long post-cure at high temperature, meaning that productivity is not fully satisfactory.
US 2006/0 166 113 proposes inorganic black pigments, particularly mixed metal oxides (MMOs), wherein it is essential that two fractions of differing granulometry be combined with each other (a first, fine fraction having an average particle size of 5-50 nm and a second, coarse fraction having an average particle size from 0.5-5.0 μm). However, these compositions do not satisfactorily meet the abovementioned requirements. Moreover, mixed metal oxides contain heavy metals not generally recognized as safe for humans and the environment.
DE 196 12 956 proposes replacing conventional chrome black by graphite, carbon black or mixtures of red, green and blue pigments. However, the first two are electrically conductive and no concrete examples of red, green or blue pigments are disclosed.
U.S. Pat. No. 5,248,576 discloses an electrically conductive resist composition combined either with a surface insulating layer of polyvinyl alcohol (Example 1) or with insulating binders (Example 2) or with dye dispersions (Example 3) as a mixture. The properties, particularly photosensitivity and cross-talk, leave something to be desired, however. In addition, the dispersion process and dispersion quality control are made significantly more difficult as a result.
Organic black colorants have also been already proposed, for example perylenes, such as C. I. Pigment Black 32. However, such previously known organic black pigments all have the disadvantage of possessing unsatisfactory jetness, particularly after high thermal stress.
U.S. Pat. No. 5,392,145 and US 2005/0 243 247 disclose LCDs with a black matrix and also the production thereof, but fail to provide any information whatsoever about the chemical nature of the black colorant. U.S. Pat. No. 5,392,145 discloses an insulating layer, which makes for additional cost and inconvenience in the manufacturing process.
WO 00/24 736 discloses the compound of the formula
which is obtained by condensation of the bisbenzofuranone of the formula
with isatin in acetic acid of unknown strength as a violet powder (Example 12b). However, there is no indication whatsoever in WO00/24 736 that black might be achievable with any of the oxobenzofuranylidenedihydroindolones disclosed. US 2003/0 083 407 criticises that the yield is suspect. In addition, no performance characteristics whatsoever are disclosed and the violet powder is in a highly aggregated form, which is not fully satisfactorily dispersible in plastics or actinically curable compositions for example.
WO 01/32 577 discloses vitreous materials, including (Example 41) a glass plate coated with tetraethoxysilane, aqueous nitric acid and the colorant of the formula
the glass plate having an absorption maximum at 760-765 nm after heating to 200° C. However, this colorant does not have a satisfactory solubility, preventing a high concentration and thus also the high colour strength, in the desired thickness, that is necessary for black matrices. Moreover, black is achieved in WO 01/32 577 via trichromatics exclusively.
There therefore remains a hitherto unfulfilled wish for black matrices for colour filters, particularly LCDs, that have high qualitative properties and permit the production of all known colour filter species of any design according to all known including particularly state of the art processes, such as inkjet printing of the colour filter and/or liquid crystal patterns.
PCT/EP 2008/059 265 is a patent application as defined in EPC Article 54(3) and PCT Rule 64.3.